1. Field of the Invention
The present invention relates generally to a self-diagnosis system for an auxiliary air control system of an internal combustion engine. More specifically, the invention relates to a self-diagnosis system for rapidly detecting malfunction of an auxiliary air control system.
2. Description of the Background Art
When a throttle valve of an internal combustion engine of automotive vehicles is closed or slightly opened, only a restricted small amount of air can pass through the main air intake passage. The air speed is low, and therefore very little vacuum is developed in the venturi. Therefore, an air intake system for an internal combustion engine of automobiles, has an auxiliary air control system including an auxiliary air passage bypassing the throttle valve in addition to the main air intake passage comprising the venturi. The auxiliary air passage includes an auxiliary air control valve provided therein to control the amount of air passed therethrough by adjusting opening angle of the control valve. This allows the idling speed of the engine to be controlled.
One such system, called an idle system, has been disclosed in the Japanese Utility Model Publication No. (Jikkai Showa) 60-15942.
Generally, a vehicle having an automatic transmission will cause substantial creep if the amount of air flow passing through the auxiliary air passage is suddenly increased due to malfunction in the auxiliary air control system, and as a result the idling speed is abnormally increased.
Therefore, self-diagnosis systems have been proposed for controlling the auxiliary air control system so as to prevent the vehicle from creeping due to malfunction of the auxiliary air control system.
The prior self-diagnosis system detects malfunction of the auxiliary air control system by comparing a target idling speed N.sub.SET with an actual idling speed N. If N&gt;N.sub.SET +500 rpm for a predetermined time, such as, for example 10 sec, a command signal for full closure of the auxiliary air control valve, is generated upon detection of malfunction thereby preventing the creep of the vehicle.
However, the prior self-diagnosis system, wherein self-diagnosis is performed by comparing the target idling speed with the actual idling speed, tends to erroneously assume abnormality in the auxiliary air control system when the vehicle is coasting with the throttle closed. This system has a further disadvantage in that it requires the aforementioned predetermined judgement time for detecting malfunction of the control system. This judgement time is relatively long.
In order to overcome the problems in the prior systems and increase the performance of the self-diagnosis systems, there has been proposed an improved self-diagnosis system which reliably detects malfunction of the auxiliary air control system within a relatively short time from just after the occurrence of malfunction until the decision of malfunction.